Posters - WGOMD Workshop on Ocean Mesoscale Eddies
CABARET in the ocean gyres: a novel high-resolution computational method for fluid mechanics 
Sergey A. Karabasov (Uni. Cambridge), Pavel S. Berloff (Imperial College), and V.M. Goloviznin
4DVAR data assimilation and Adjoint-based sensitivity analysis in an eddy resolving ocean model of the Peruvian Current System
Aneesh Subramanian (SCRIPPS), A. J. Miller (SCRIPPS), B. D. Cornuelle (SCRIPPS), and E. Di Lorenzo (Gerogia Inst. Tech)
Tropical instability waves and their impact on the tropical atmosphere in a high resolution coupled climate model
Len Shaffrey (Uni. Reading)
Modification of gyre circulation by sub-mesoscale physics
M. Lévy (LOCEAN-IPSL), P. Klein (LPO-IFREMER), A.-M. Tréguier (LPO-IFREMER), D. Iovino (LOCEAN-IPSL), G. Madec (LOCEAN-IPSL), S. Masson (LOCEAN-IPSL) and K. Takahashi (ESP)
Diagnosis of eddy fluxes in a hierarchy of high resolution models of the North Pacific
Hiroyuki Tsujino (MRI)
Toward a North Pacific simulation resolving submesoscale structures
Hideharu Sasaki (JAMSTEC)
Meso-scale eddy processes in North Pacific Subtropical Mode Water formation
Ivana Cerovecki (SCRIPPS)
Bottom Water Formation in the southern Weddell Sea and Western Ross Sea
Qiang Wang (AWI)
The Role of Internal Waves and Turbulence in the Meridional Overturning Circulation of the Southern Ocean
Maria Broadbridge (NOC), Alberto C. Naveira Garabato (NOC), David A. Smeed (NOC), David P. Stevens (UEA) and Raffaele Ferrari (MIT)
Eddy-induced mixing in the Southern Ocean
Alberto Naveira Garabato (NOC)
Weak Along-Isopycnal and Strong Vertical Mixing in the Southern Ocean Interior
Jan Zika, Trevor J. McDougall, and Bernadette M. Sloyan (CSIRO)
The Antarctic Circumpolar Current across the AR4 models towards a common denominator
Till Kuhlbrodt (Uni. Reading)
Ocean heat advection in the South-East Pacific in a high-resolution coupled GCM
Thomas Toniazzo (Uni. Reading), Roberto Mechoso (UCLA), Len Shaffrey (Uni. Reading), and Julia Slingo (Uni. Reading/Met Office)
The role of mesoscale eddies in the rectification of the Southern Ocean response to climate change
Riccardo Farneti, Tom L. Delworth, Anthony J. Rosati, W. Anderson, H.C. Lee, R. Pacanowski, R. Zang and F. Zeng (NOAA/GFDL)
The role of buoyancy and mechanical forcing in determining the residual circulation in the Southern Ocean
Gualtiero Badin and Richard G. Williams (Liverpool Uni.)
The influence of standard modes of variability on an eddy-resolving Southern Ocean model
Marshall Ward (ANU)
The Role of Mesoscale Eddies on Circumpolar Transport and Ocean Carbon Storage
Dave Munday (Uni. Oxford)
Eddy life cycles and ocean storm tracks
Richard G. Williams (Uni. Liverpool), Chris Wilson (Uni. Liverpool and POL) and Chris W. Hughes (POL)
The control of polar haloclines by along-isopycnal diffusion in climate models
Willem Sijp (UNSW)
Deep Cyclogenesis By Synoptic Eddies Interacting With A Seamount
Andrew Greene (Uni. Rhode Island)
Non QG effects in rotating stratified turbulence
Guillaume Roullet (IFREMER)
Maintenance of the mean kinetic energy in the global ocean
Hidenori Aiki (JAMSTEC), Kelvin J. Richards (Univ. of Hawaii) and H. Sakuma (JAMSTEC)
Meso-scale eddy parameterizations forward APE cascade and diapycnal mixing
Remi Tailleux (Uni. Reading)
An Estimate of the Isopycnal Diffusivity Tensor Based on Lagrangian Particle Statistics
Stefan Riha (IFM-GEOMAR), Alexa Griesel (SCRIPPS), Sarah T. Gille (SCRIPPS) Carsten Eden (IFM-GEOMAR)
Scaling eddy parameterizations with locally eddy resolving ocean models
Robert Hallberg (NOAA/GFDL) and Alistair Adcroft (Princeton Uni.)
An observational test of diffusive heat flux parameterizations in the ocean and the atmosphere
Arnaud Czaja (Imperial College)
Influence of diabatic processes on mesoscale eddies and ocean-atmosphere coupling
Ute Hausmann and Arnaud Czaja (Imperial College)
Eddy Diagnostics from Altimetry
Chris W. Hughes (POL)
Comparison of global eddying models and current meters
Robert Scott (Uni. Texas and NOC) and Brian Arbic (FSU)
Estimates of bottom flows and bottom boundary layer dissipation of the oceanic general circulation from global high-resolution models
Brian Arbic (FSU) et al.
Internal waves generation propagation and induced mixing in OGCMs
F. Gouillon and E.P. Chassignet (COAPS/FSU)
The temporal residual-mean velocity evaluated by using output of an eddy resolving ocean model
Y. Tanaka (FRCGC JAMSTEC) and H. Hasumi (CCSR Uni. Tokyo)
Quantifying North Brazil Current rings heat and mass transport using Lagrangian tracers
Roberto A. Ferreira De Almeida and Paulo Nobre (CPTEC/INPE)
Lagrangian blobs of buoyancy embedded in Eulerian models: a framework to parameterise vertical and downslope motion of gravitationally unstable water parcels
Micheal Bates (UNSW), Matthew England (UNSW), Stephen Griffies (NOAA/GFDL) and Alistair Adcroft (Princeton Uni.)
Formation of Multiple Zonal Jets in the Oceans
Pavel S. Berloff (Imperial College), Igor Kamenkovich (WHOI) and Joe Pedlosky (RSMAS)
Eddy overturning across a shelf edge front
Vigdis Tverberg (Norwegian Polar Institute) et al.
Kilometer-scale turbulent activity over a wide continental shelf
Xavier Capet (IFREMER)
Impact of a Barotropic Boundary Current on Eddies in a Convective Basin
Julie Deshayes, Michael Spall and F. Straneo (WHOI)
Eddy-mean flow interactions in western boundary current jets
S.N. Waterman (WHOI, now NOC), S.R. Jayne (WHOI) and N.G. Hogg (Cornell Uni.)
The Gulf Stream warm core ring interaction with continental shelf and slope
Jun Wei (MIT) and Dong-Ping Wang (Stony Brook University)
On the Impact of Mesoscale Eddies on the Biological Production in the California Current System
Matthias Munnich (ETH Zurich)
The Variability of Respiration Versus Photosynthesis and the Influence of Mesoscale Dynamics
Beatriz Mouriño-Carballido (Uni. Vigo)
A Low Power Ultra Violet Spectrophotometer on a Towed Platform for High Resolution Measurement of Nitrate Concentrations within Open Ocean Mesoscale Features
Rosalind Pidcock, Stuart Painter, Meric Srokosz, John Allen, Matt Mowlem, David Hydes and Richard Sanders (NOC)
A Low Power Ultra Violet Spectrophotometer on a Towed Platform for High Resolution Measurement of Nitrate Concentrations within Open Ocean Mesoscale Features
Rosalind Pidcock, Stuart Painter, Meric Srokosz, John Allen, Matt Mowlem, David Hydes and Richard Sanders (NOC)
We present data from the first successful integrated deployment of an ultra-violet spectrophotometer on board a towed vehicle to measure in-situ nitrate concentrations in the surface ocean. The sensor monitors UV absorbance in seawater at 220 nm and provides instantaneous, near continuous nitrate measurements with a detection limit of 0.2 mmol L-1 NO3. The data were collected as part of a high resolution, mesoscale survey in the Iceland Basin in summer 2007. Early in the cruise, satellite imagery showed the appearance of an eddy dipole structure within our survey area. Using near-real time satellite imagery together with Acoustic Doppler Current Profiler (ADCP) data, short, targeted surveys were made of the developing eddy feature using the SeaSoar towed vehicle to make continuous measurements of hydrographic and biogeochemical parameters. For the first time, we are able to make high-resolution observations of nitrate concentration alongside hydrographic parameters and changes in dissolved oxygen concentration; the latter determined with a Seabird SBE43 sensor as a further component of the integrated instrument package on SeaSoar. The value of concomitant high resolution hydrographic, oxygen and nitrate datasets is to be able to make considerably more detailed interpretations of the fine-scale variability within the water column than have been possible in the past using traditional CTD survey techniques. From this, we can gain a better understanding of the complex interplay between (sub)mesoscale dynamics and the underlying factors behind spatial variability of primary production in the upper ocean.
The Variability of Respiration Versus Photosynthesis and the Influence of Mesoscale Dynamics
Beatriz Mouriño-Carballido (Uni. Vigo)
Respiration is assumed to be much less variable than photosynthesis in aquatic ecosystems. An analysis of nine years of data from the NW subtropical Atlantic reveals that variability in heterotrophic processes associated with submesoscale features has a major impact on the balance between photosynthesis and respiration. Higher indirect estimates of net community production (NCPe) are associated with the center of Mode Water Eddies (MWE) and frontal regions between cyclonic and anticyclonic eddies (CA). The increase in NCPe observed at the center of MWE is driven mainly by an increase in autotrophic production whereas in CA enhanced NCPe rates are the result of an important reduction in bacterial respiration. Both features also exhibit a decrease in nitrate concentration consistent with nutrient consumption and relative increases in oxygen anomaly and particulate and dissolved organic carbon in the upper 200m. Plankton community composition in CA and MWE is characterized by the reduction in bacterial biomass and the dominance of Prochlorococcus and Synechococcus in CA and diatoms and dinoflagellates in MWE. Contrary to a common assumption these results show for the first time that in ecosystems influenced by submesoscale dynamics respiration can be as variable as photosynthesis.
On the Impact of Mesoscale Eddies on the Biological Production in the California Current System
Matthias Munnich (ETH Zurich)
Open-ocean studies indicate that mesoscale eddies enhance the supply of nutrients in surface waters and thus increase biological productivity. This effect is most pronounced in low nutrient regions of the world ocean. Here we report observation- and model-based results indicating a different response of biological production to mesoscale dyanmics in Eastern Boundary Upwelling Systems. Our study of the California Current System with an eddy-resolving ocean circulation - ecosystem model indicates that the net effect of eddies in this highly dynamic nutrient-rich coastal upwelling region is to transport warm nutrient-depleted waters closer towards shore near surface. This transport induces the subduction of high-nutrient waters below the euphotic zone and where it is moved off-shore. Both effects lead to a net reduction of the nutrient supply to the surface and thus tend to lower biological productivity.
The Gulf Stream warm core ring interaction with continental shelf and slope
Jun Wei (MIT) and Dong-Ping Wang (Stony Brook University)
The roles of Gulf Stream warm core rings WCRs on the shelf and slope water exchange remain largely unexplored. The chlorophyll concentrations are enhanced on the shelfbreak along the MAB and Georges Bank during the spring transition. The warm ring influence often is cited but no clear physical mechanism has been identified. This study investigates the large-scale structures of WCRs the ring - shelfbreak interaction and the associated small-scale feature. With the aid of SSHA Sea Surface Height Anomaly SST Sea Surface Temperature and GLOBEC drifter data the repeat Oleander ADCP transects are mapped to examine the WCR flow structures and their interaction with the Gulf Stream and the shelf and slope seas. Furthermore a three-dimensional primitive-equation model POM is used to examine the interaction of warm core rings with shelf/slope. The model ring is initialized with an axisymmetric Gaussian-type anticyclonic eddy placing far from the boundary to allow the ring to freely propagate towards the shelf/slope. Upon collision with the shelf and slope the ring structure and trajectory undergoes a more complicated adjustment. The ring typically becomes stalled and bounces on- and off the shelf/slope. Cyclones with associated strong upwelling are generated near the shelfbreak north of the ring. Cyclones and anticyclones also are generated at the periphery of the ring. These small-scale but energetic features may be fundamental mechanisms to the biological production.
Eddy-mean flow interactions in western boundary current jets
S.N. Waterman (WHOI, now NOC), S.R. Jayne (WHOI) and N.G. Hogg (Cornell Uni.)
The Kuroshio Extension System Study (KESS) is a large-scale observational program of the Kuroshio Extension investigating the processes that govern the jet's variability and the relation between the jet and its recirculation gyres. Motivated by the KESS observations, we examine the nature and the importance of mesoscale eddy-mean flow interactions in Western Boundary Current (WBC) jet systems.
We use the KESS observations to design the set-up of an idealized quasi-geostrophic numerical model and use it to study the role of eddy-mean flow interactions in a baroclinic, unstable, boundary-forced jet. We find that in this simplified configuration, in a parameter regime relevant to the Kuroshio Extension and the Gulf Stream, the unstable jet evolves through the shedding of eddies as it flows through the domain until it reaches stability, and the nonlinear eddy fluxes play one of two distinctive roles depending on the downstream location. Upstream of the jet's stabilization point, the eddies act to stabilize the jet through a diffusive-like, down-gradient potential vorticity flux. However downstream of this stabilization, the sense of this eddy flux reverses, and now acts to drive the recirculations through an anti-diffusive, up-gradient flux. Properties of this eddy-driven time-mean circulation are related to the stability properties of the upstream jet that is the source of the eddy variability.
We next test the relevance of these idealized results to actual oceanic WBC jet systems through analysis of the KESS observations. We show that these have several dynamically significant signatures in common with the idealized model's predictions. These similarities suggest that the simplified physics we have studied is a potentially relevant and useful means to understand the role of mesoscale eddies in WBC jet systems.
Impact of a Barotropic Boundary Current on Eddies in a Convective Basin
Julie Deshayes, Michael Spall and F. Straneo (WHOI)
Dense water formation is an essential component of the current climate as it is associated with a substantial transfer of heat from the ocean to the atmosphere. It mostly occurs in the interior of semi-enclosed basins, where the surface heat loss is compensated for by heat advection by the boundary current, which penetrates into the interior of the basin via eddies. We investigate whether and how the turbulent heat fluxes associated with the eddies depend on the barotropic component of the boundary current (hereafter bbc) by running several eddy-permitting (5km horizontal resolution) simulations with MITgcm. The net turbulent heat fluxes are directly related to the net surface heat loss in the basin interior, which is the same for all simulations. Nevertheless, the horizontal and vertical structure of the turbulent heat fluxes are affected by changes in the bbc at the basin inflow. In particular, the turbulent heat fluxes reach deeper as the bbc increases, because of downwelling along the sloping sides of the basin that brings heat at depth in the boundary current. The turbulent heat fluxes are also more intense as the bbc increases, presumably due to barotropic instabilities Finally, as the bbc increases, the net oceanic heat transport into the basin associated with the overturning circulation increases, while that related to the horizontal circulation decreases.
Kilometer-scale turbulent activity over a wide continental shelf
Xavier Capet (IFREMER)
Tracer satellite images suggest that continental shelves undergo occasional episodes of widespread turbulent activity with scales in the range 1-10 km (aka submesoscale). This activity is characterized by frontal features such as filaments and small-scale eddies. A set of realistic numerical solutions for the Northern Argentinian shelf is being analyzed for its submesoscale turbulence. We are able to identify its primary underlying generation mechanism, to explain the seasonal modulation of its intensity (low in spring and summer high in fall and winter), and also to quantify its impact on the shelf dynamics.
Eddy overturning across a shelf edge front
Vigdis Tverberg (Norwegian Polar Institute) et al.
Eddy overturning across a shelf edge front has been investigated in a simplified model setup comprising an along-slope uniform shelf and slope with a slope current and initiated with temperature and salinity data collected off Kongsfjorden at Spitsbergen. The model results illustrate how eddy overturning act towards flattening cross-frontal density gradients. Clockwise eddy overturning combined with atmospheric cooling may have lead to an efficient cooling of the West Spitsbergen Current during the late winter seasons of 2007 and 2008.
Formation of Multiple Zonal Jets in the Oceans
Pavel Berloff (Imperial College), Igor Kamenkovich (WHOI) and Joe Pedlosky (RSMAS)
Multiple alternating zonal jets observed in the ocean are studied with an idealized quasigeostrophic model with the background flow imposed. Formation of the jets is governed by a spatially nonlocal mechanism that involves basin-scale instabilities. Energy of the background flow is released to the primary unstable mode with long meridional and short zonal lengthscale. This mode undergoes secondary instability that sets meridional scale of the multiple zonal jets. In a zonal channel eddies generated by the instabilities maintain several weakly damped annular modes that significantly modify the jets and feed back on the primary instability. It is found that the jets are driven by the mixed barotropic-baroclinic dynamics and maintained by either Reynolds or form stress forcing depending on the direction of the background flow. The underlying dynamical mechanism is illuminated both with statistical analysis of the nonlinear equilibrium solutions and with linear stability analysis of the flow components. Finally we find that the jets are associated with alternating weak barriers to the meridional material transport but locations of these barriers are not unique.
Lagrangian blobs of buoyancy embedded in Eulerian models: a framework to parameterise vertical and downslope motion of gravitationally unstable water parcels
Micheal Bates (UNSW), Matthew England (UNSW), Stephen Griffies (NOAA/GFDL) and Alistair Adcroft (Princeton University)
A Lagrangian framework for representing open ocean convection and near boundary convection is presented. The Lagrangian framework arbitrarily re-labels parcels of fluid in a grid cell that can then be treated pseudo-independently from the gridded model. The parcels, or "blobs" can then be moved around in three dimensions and interact with the Eulerian model in an arbitrary manner. In doing so, blobs can effect transport of properties vertically through the water column and laterally.
The technique is not a parameterisation, but rather a framework in which a multitude of parameterisations may be implemented. As such, it is possible to recover parameterisations which are analagous to many existing and commonly used parameterisations, for both upright convection and downslope flows. The framework operates in three basic steps. (1) Water and tracer are conservatively transferred from the Eulerian model to the Lagrangian model. (2) Lagrangian "blobs" then transport these properties according to the specific parameterisation, during which properties may be exchanged between the Lagrangian and Eulerian models. (3) The water and tracer is conservatively transferred back to the Eulerian model, via mechanisms that depend on details of the parameterisation.
One of the most promising aspects of this approach is that it permits dynamics, allowing the blobs to move and respond to the bulk ocean properties and physics in a realistic way. Furthermore, the framework permits pseudo-non-hydrostatic dynamics, which essentially allows the blob to sink (either in the open ocean, or along topography) if it is surrounded by water that is less dense. Another promising aspect of the framework is that it permits explicit entrainment and detrainment by the transfer of properties from an Eulerian grid box to a Lagrangian blob and vice versa. This framework potentially allows for a more complete and physically based representation of open ocean convection and downslope flows.
Quantifying North Brazil Current rings heat and mass transport using Lagrangian tracers
Roberto A. Ferreira De Almeida and Paulo Nobre (CPTEC/INPE)
In this work we describe a technique for the programmatic identification of water parcels that are part of rings produced by the North Brazil Current NBC. We use the Gradient Pattern Analysis GPA asymmetry operator to characterize the Lagrangian flow as laminar or quasi-periodic Assireu et al. 2002 water parcels composing NBC rings exhibit a quasi-periodic behavior that breaks the velocity vector asymmetry resulting in lower values for the coefficient. We are now combining this technique with Lagrangian tracers Marsh et al. 2000 De Vries and Döös 2001 in order to quantify the interhemispheric heat and mass transport that is performed by NBC rings in a fully coupled numerical simulation.
The temporal residual-mean velocity evaluated by using output of an eddy resolving ocean model
Y. Tanaka (FRCGC JAMSTEC) and H. Hasumi (CCSR Uni. Tokyo)
The temporal residual-mean TRM velocity rather than the usual Eulerian time-mean velocity represents transport of water mass in an eddy-rich ocean. The TRM velocity is the sum of an eddy-induced and the time-mean velocity. It is important to evaluate a vertical component of the TRM velocity to investigate subduction of water masses in an eddy-rich ocean. By using output of an eddy resolving ocean model with about 10 km horizontal resolution the vertical component of the TRM velocity in the Southern Ocean is evaluated. The obtained TRM velocity shows strong downward transport at the Brazil-Malvinas Current confluence. The eddy-induced velocity dominates the TRM velocity. This downward eddy-induced transport seems to explain the injection of low potential vorticity water of Malvinas Current into the Atlantic subtropical gyre and the formation of the low salinity tongue of Antarctic Intermediate Water in the Atlantic Ocean.
Internal waves generation propagation and induced mixing in OGCMs
F. Gouillon and E.P. Chassignet (COAPS/FSU)
Recent observations and numerical experiments suggest that energy from the barotropic tides is the main contributor for the deep ocean mixing by the process of internal wave breaking. Here the process of internal wave generation and propagation is first examined for an idealized configuration isolated Gaussian topography using an analytical solution and two numerical models the HYbrid Coordinate Ocean Model HYCOM and the Regional Ocean Modeling System ROMS. The main goals of this study are 1 to investigate the internal wave representation in these models as a function of a wide model grid spacing range 500 m to 100 km and 2 to quantify the spurious diapycnal mixing associated with the transport of density in the fixed coordinate ocean model ROMS. The results show that the choice of model grid resolution is crucial to properly simulate the internal waves. In the coarser grids the resulting baroclinic velocities are weaker and critical locations where internal waves are generated can be removed. This leads to inaccuracy in capturing the tidal conversion process and thus misrepresentation of energetic smaller scale motions. The refinement of the grid resolution rapidly converges to the analytical solution from which a grid spacing is determined that provides sufficient accuracy of the numerical solution. Some estimates of the numerically induced vertical mixing in ROMS are provided for these model configurations.
Estimates of bottom flows and bottom boundary layer dissipation of the oceanic general circulation from global high-resolution models
Brian Arbic (FSU)
We 1) compare the bottom flows of three existing high-resolution global simulations of the oceanic general circulation to near-bottom flows in a current-meter database and 2) estimate from the simulations the global energy dissipation rate of the general circulation by quadratic bottom boundary layer drag. We utilize a 1/32 degree data-assimilative run of the Naval Research Laboratory Layered Ocean Model NLOM a 1/32 degree non-assimilative run of NLOM and a 1/10 degree non-assimilative run of the Parallel Ocean Program POP z-level ocean model. While NLOM has greater horizontal resolution POP has greater vertical resolution and a more standard treatment of bottom topography. Generally speaking the simulations have some difficulty matching the flows in individual current-meter records. However averages of model values of the time-average of the cube of bottom velocity which is proportional to the dissipation rate computed over all the current-meter sites agree to within a factor of 2.7 or better with averages computed over the current meters themselves at least in certain depth ranges. The models therefore likely provide reasonable order-of-magnitude estimates of areally integrated dissipation by bottom drag. Global dissipation rates range from 0.14 to 0.65 TW suggesting that bottom drag represents a substantial sink of the 1 TW wind-power transformed into geostrophic motions. Work is in press for Journal of Geophysical Research co-authored by Jay F. Shriver Patrick J. Hogan Harley E. Hurlburt Julie L. McClean E. Joseph Metzger Robert B. Scott Ayon Sen Ole Martin Smedstad and Alan J. Wallcraft
Comparison of global eddying models and current meters
Robert Scott (Uni. Texas and NOC) and Brian Arbic (FSU)
We compare kinetic energy KE statistics from three eddying global ocean general circulation models with observations from over 2000 current meter moorings distributed throughout the World Ocean. While the scatter is large suggesting model skill at particular mooring sites is low there is an overall agreement between the observed and simulated KE. Stratifying the comparison by depth we find there is underestimation of simulated KE that gets worse at greater depths. We speculate that this is related to inhibited barotropization resulting from an under resolved inverse energy cascade.
Eddy Diagnostics from Altimetry
Chris Hughes (POL)
Altimetry is often used as a measure of eddy activity usually plotted as standard deviation of sea level or eddy kinetic energy with a kind of tacit assumption that eddies are all effectively doing the same thing but with different energy levels. Here I present a range of diagnostics based on satellite altimetry which demonstrate that the energy takes very different forms and plays different roles in different places with large variations over sometimes very small length scales. This is apparent from plots of sea level anomaly and relative vorticity propagation speed vorticity flux divergence skewness kurtosis eddy-mean flow energy flux and the spectrum of sea level variability which will be presented in this poster.
Influence of diabatic processes on mesoscale eddies and ocean-atmosphere coupling
Ute Hausmann and Arnaud Czaja (Imperial College)
Sea Surface Temperature (SST) anomalies are generally observed to be damped by turbulent air-sea heat fluxes in the midlatitudes (Frankignoul, 1985). As shown by various theoretical and modelling studies (Bretherton, 1982; Rahmstorf and Willebrand, 1995), this negative heat flux-SST feedback is expected to increase towards smaller scales and will therefore attenuate the SST anomalies at the origin of ocean-atmosphere coupling on the mesoscale (~100 km). An estimate of the strength of this feedback is therefore crucial to determine
(i) how important ocean-atmosphere coupling is for mesoscale eddies, and
(ii) whether there is a significant contribution from the mesoscale to the total (ocean plus atmosphere) heat transport in midlatitudes.
Here we present an estimate for the strength of this damping on the mesoscale built from satellite observations. From the AMSR-E high-resolution microwave SST dataset we estimate typical time-scales for the persistence of mesoscale SST anomalies from the e-folding time of their autocorrelations. We find that mesoscale SST anomalies persist only around 15 to 40 days, much less than larger scale SST anomalies that persist between 1 and 3 months as estimated from the lower resolution Reynolds et al.(2002) dataset. Combined with a mixed layer depth climatology, the estimates of SSTA persistence give a crude estimate for the damping coefficient. For the large-scale dataset it ranges on average around 50 W/m2K, on the mesoscale it is consistent with much larger values (around 100 W/m2K or higher).If mesoscale SST anomalies are indeed quickly damped on the mesoscale, they should persist significantly less than the associated eddies, visible by their SSH anomaly signatures. Observed highresolution SSH anomalies from AVISO show indeed a longer persistence than SST anomalies, a difference that is most clearly marked in North Atlantic regions of high mesoscale activity. From quiescent to highly energetic oceanic regions correlations between mesoscale SSH and SST signatures systematically increase, suggesting that SST anomalies are controlled by large-scale processes in the former and firmly associated to mesoscale eddies in the later. The observed decrease of SST anomaly persistence towards regions of higher mesoscale energy is therefore consistent with the expected increase in damping strength towards the smaller scales.
An observational test of diffusive heat flux parameterizations in the ocean and the atmosphere
Arnaud Czaja (Imperial College)
An important assumption made when parameterizing eddy fuxes as diffusive is that of scale separation between the eddies and their environment. We show here that this separation can be quantifed by a non dimensional number ε (<< 1 when scale separation is obeyed, >> 1 when it isn't) and we estimate its value for Southern Hemisphere surface temperature in the ocean (AMSR-E, 1=4° spatial resolution) and the atmosphere (NCEP reanalysis). In agreement with previous studies (e.g., Held, 1999) it is found that a diffusive parameterization of the eddy heat flux near the sea surface is warranted for the atmosphere, with 90 % of gridpoints having ε < 0:5. In the ocean, however, the distribution of ε typically shows a much slower decay with ε. The situation is particularly problematic on the poleward side of the Antarctic Circumpolar Current (ACC) where a diffusive parameterization is not supported by observations. The ACC is in a regime in between the latter and that of the atmosphere.
Scaling eddy parameterizations with locally eddy resolving ocean models
Robert Hallberg (NOAA/GFDL) and Alistair Adcroft (Princeton Uni.)
There is not yet an eddy resolving global ocean model: global ocean models often do a reasonable job of resolving the largest mesoscale eddies in one part of the domain while failing to resolve the dominant eddy scales in other parts of the domain. For example tropical eddies in the open ocean often have much larger spatial scales than coastal or high-latitude eddies. Where the eddy scales are not resolved their effects need to be parameterized but since many eddy parameterizations strongly suppress explicit eddies it is desirable to avoid the parameterizations where they are not needed. This poster presents a scaling for the intensity of parameterized eddy effects based on the ratio of the grid spacing to the first-mode baroclinic deformation radius. When combined with a parameterization for the intensity of eddy activity based on a two-dimensional diagnostic equation of the Mesoscale Eddy Kinetic Energy this scaling appears to be particularly promising. The outcome of this approach is demonstrated with results from a 1-degree global ocean model and a 1/8-degree Mercator global ocean model that both use the same eddy parameterization with identical nondimensional coefficients but give very different parameterized lateral thickness diffusivities and exhibit explicit mesoscale eddies where they are resolved.
An Estimate of the Isopycnal Diffusivity Tensor Based on Lagrangian Particle Statistics
Stefan Riha (IFM-GEOMAR), Alexa Griesel (SCRIPPS), Sarah T. Gille (SCRIPPS) Carsten Eden (IFM-GEOMAR)
Lagrangian particle statistics from an an eddy-resolving model of the North Atlantic are used to estimate the horizontal and vertical distribution of the complete isopycnal eddy diffusivity tensor. First the Lagrangian data are processed using different computational methods to assess the sensitivity to the choice of method. Second the estimated numerical diffusivities are compared to available observational data and it is found that the model's diffusivities generally correspond well to observations. Third the correspondence of the lateral and vertical distribution of eddy kinetic energy and Lagrangian length scales in the classical mixing length ansatz to the estimated scalar isopycnal diffusivities is evaluated. Furthermore it is shown how the horizontal shear of the mean currents and the associated shear dispersion as observed near western boundaries or near the equator is related to strongly anisotropic symmetric isopycnal diffusivity. We also discuss how the symmetric and antisymmetric parts of the complete isopycnal diffusivity tensor could be interpreted in a physically meaningful context.
Meso-scale eddy parameterizations forward APE cascade and diapycnal mixing
Remi Tailleux (Uni. Reading)
Meso-scale eddy parameterizations are now widely recognized as a crucial component of any realistic numerical model of the oceans. Many questions remain however regarding the fundamental nature of such a parameterization in relation with the global energetics of the oceans and in particular about its link with irreversible diffusive mixing. Physically meso-scale eddy parameterizations act as a net sink of available potential energy without affecting the background stratification the irreversible modifications of the latter being treated separately. Here we show that from the viewpoint of the global energetics of the oceans the inviscid dissipation of available potential energy and the irreversible mixing of the background stratification must be intimately linked. Physically the inviscid dissipation of APE must be associated with the forward APE cascade which is linked with the irreversible diffusive mixing associated with diapycnal mixing in the oceans. Physically therefore meso-scale eddy parameterization have to be linked with the parameterization of diapycnal mixing. This poster presents the key elements to achieve this goal which involves the APE cascade the mixing efficiency local APE budget and so on.
Maintenance of the mean kinetic energy in the global ocean
Hidenori Aiki (JAMSTEC), Kelvin J. Richards (Univ. of Hawaii) and H. Sakuma (JAMSTEC)
To understand the state of the global ocean the authors analyze the budget of mean kinetic energy KE in a high-resolution numerical simulation. The analysis identifies two separate energy flows resulting from the linkage of work elements in the energy cycle namely the work linkage of wind-induced Ekman transport and the work linkage of dissipation-induced Ekman transport. The transport is separated further into barotropic and baroclinic components. The global work of the barotropic pressure gradient - which is connected to the work of bottom form stress and then to the budget of potential energy - is found to be nearly zero. Thus the budget of barotropic KE in the global ocean is maintained by both wind- and dissipation-induced barotropic Ekman transports which are of the same strength with opposite sign. It is also found that the work of wind forcing on the barotropic component of the Antarctic Circumpolar Current ACC is canceled by the combined effect in equal measure of the dissipation process and the conversion to potential energy. Thus the state of the simulated ACC is characterized by an equal blend of the dissipative and nondissipative theories for the barotropic dynamics of the ACC. On the other hand the global budget of baroclinic KE is maintained mainly by wind-induced rather than dissipation-induced baroclinic Ekman transport as is expected. The relative importance of the barotropic and baroclinic dynamics in the global ocean is quantified by the ratio about 13 between the barotropic wind work and the baroclinic wind work.
Non QG effects in rotating stratified turbulence
Guillaume Roullet (IFREMER)
Very high resolution simulations of rotating stratified turbulence using a PE model in a realistic regime reveal a dominance of cyclones from the surface to as deep as 1000m. Consequently there is a tendancy for upward isopycnal displacement. While this asymetry could be explained in terms of frontal dynamics in the upper layers the mechanism is certainly different in the interior where the Rossby number is very small less than 0.1 and therefore gradient wind balance is pretty accurate. We postulate that anharmonic APE due to the curvature of the mean stratification could be a good candidate to rationalize this asymetry.
Deep Cyclogenesis By Synoptic Eddies Interacting With A Seamount
Andrew Greene (Uni. Rhode Island)
In 2004 a multi-institutional collaborative project was launched titled the Kuroshio Extension System Study KESS. One of the goals of KESS has been to understand the processes coupling the upper ocean and deep depth-independent circulation. One working hypothesis was that meandering of the Kuroshio Extension couples the baroclinic front to deep eddies via baroclinic instability which in turn steer the Kuroshio Extension. While this holds true for some events there are cases in which a seemingly spontaneous spin-up of an abyssal cyclone occurs without the baroclinic stretching produced by jet meandering. The authors argue that in the deep Kuroshio Extension the strong cases of vorticity generation and cyclone development are related to topographic stretching and potential vorticity conservation when water columns are advected off isolated seamounts in the region. These strong deep eddies have cyclonic vorticities exceeding 20% of f and were detected with an array of bottom current and pressure measurements. Daily maps showed these deep eddies developed locally. Current meter records near the flanks of seamounts exhibited bursts of eddy kinetic energy when bandpass-filtered between the inertial period and 8 days suggestive of associated submesocale activity along with the cyclogenetic process.
The control of polar haloclines by along-isopycnal diffusion in climate models
Willem Sijp (UNSW)
Increasing the value of along-isopycnal diffusivity in a coupled model is shown to lead to enhanced stability of North Atlantic Deep Water (NADW) formation with respect to freshwater (FW) perturbations. This is because the North Atlantic (NA) surface salinity budget is dominated by upward salt fluxes resulting from winter convection for low values of along-isopycnal diffusivity whereas along-isopycnal diffusion exerts a strong control on NA surface salinity at higher diffusivity values. Shutdown of wintertime convection in response to a FW pulse allows the development of a halocline responsible for the suppression of deep sinking. In contrast to convection isopycnal salt diffusion proves a more robust mechanism for preventing the formation of a halocline as surface freshening leads only to a flattening of isopycnals leaving at least some diffusive removal of anomalous surface FW in place. As a result multiple equilibria are altogether absent for sufficiently high values of isopycnal diffusivity. Furthermore the surface salinity budget of the North Pacific is also dominated by along-isopycnal diffusion when diffusivity values are sufficiently high leading to a breakdown of the permanent halocline there and the associated onset of deep water formation.
Eddy life cycles and ocean storm tracks
Richard G. Williams (Uni. Liverpool), Chris Wilson (Uni. Liverpool and POL) and Chris W. Hughes (POL)
Eddies have characteristic life cycles which affect the direction of any associated tracer fluxes eddy fluxes are directed down the tracer gradient whenever there is a Lagrangian increase in tracer variance as well as if there is strong dissipation of tracer variance. In the atmosphere the life cycle of eddies is reflected in the presence of storm tracks at the entrance of the storm track eddies grow providing a down-gradient heat flux and accelerating the mean flow eastward while at the exit and downstream of the storm track the eddies decay and instead provide a westward acceleration. In the ocean there might be analogous features along the Antarctic Circumpolar Current where there is high eddy kinetic energy high Eady growth rate and down-gradient eddy heat fluxes. In particular altimetric diagnostics suggest localised dipole patterns in eddy vorticity forcing where the eddies first provide an eastward acceleration of the mean flow and then a westward acceleration this eddy forcing can reach 1/2 the magnitude of the planetary vorticity advection by the time-mean flow.
The Role of Mesoscale Eddies on Circumpolar Transport and Ocean Carbon Storage
Dave Munday (Uni. Oxford)
A good candidate to help explain the large difference in glacial-interglacial CO2 levels is changes in ocean circulation and/or stratification. In essence changes in the depth of the global ocean pycnocline may lead to increased ocean storage of carbon dioxide by increasing the volume of cold water in the deep ocean. Gnanadesikan's model of the global pycnocline indicates the effect that Southern Ocean wind forcing and eddy transport tidally-driven diapycnal mixing and northern hemisphere sinking may have upon the mean pycnocline depth giving an indication of the effect that these processes may have on ocean carbon storage. Furthermore the sensitivity to Southern Ocean processes suggests a crucial link to the transport and dynamics of the Antarctic Circumpolar Current (ACC) and eddy transports which play a role in setting the stratification of the ocean. We use the MIT general circulation model (MITgcm) to investigate the sensitivity of the global pycnocline depth and circumpolar transport to a variety of forcing permutations. The experiments are conducted in an idealised sector domain spanning 20 degrees in longitude and 120 degrees in latitude. This abbreviated domain makes it possible to achieve integration lengths of O10 000 years at grid spacings ranging from coarse resolution with parameterised eddies to eddy permitting resolutions. The forcing permutations investigated include multiples of the wind forcing and an altered northern surface salinity field allowing for alterations of the northern sinking and meridional overturning without gross changes in other forcing fields. Preliminary results using the full biogeochemistry of MITgcm allowing the ocean carbon storage to be directly diagnosed and experiments using eddy-resolving resolutions are also presented. We anticipate that these experiments will be valuable for elucidating the role of the Southern Ocean in glacial cycles.
The influence of standard modes of variability on an eddy-resolving Southern Ocean model
Marshall Ward (ANU)
Satellite altimeter data of the Southern Ocean (SO) reveal an anomalous peak in eddy kinetic energy (EKE) in the Antarctic Circumpolar Current (ACC) in 2000-2002. This peak has been attributed to a delayed response to an earlier peak in the Southern Annular Mode (SAM) and its associated circumpolar eastward winds that occurred around 1998 where the delay is due to the formation and adjustment of the eddy field associated with the increased winds Meredith & Hogg 2006. A more recent analysis reveals that the EKE response varies regionally with the strongest response in the Pacific and it has been suggested that this variability is due to the additional influence of ENSO. The 2000-2002 peak in EKE is therefore attributed to the coincident peak in SAM and ENSO 2-3 years earlier and that the EKE response was weaker in past years when modes were out of phase Morrow & Pasquet 2008. We investigate this issue by applying SAM-like and ENSO-like wind forcings to Q-GCM the eddy-resolving model used in Meredith & Hogg and conifigured for the Southern Ocean. We analyze the EKE response to each individual forcing as well as a simultaneous forcing of the two both in and out of phase. From these results we are able to quantify both the global and regional response to each forcing and the degree to which each mode is responsible for the EKE strength and distribution across the ACC.
The role of buoyancy and mechanical forcing in determining the residual circulation in the Southern Ocean
Gualtiero Badin and Richard G. Williams (Liverpool Uni.)
The effect of buoyancy and mechanical forcing on the residual circulation in the Southern Ocean is examined in two different ways. First, the rates of water-mass transformation and formation are estimated using air-sea fluxes of heat and freshwater using the isopycnal framework developed by Walin, which is applied to the NOCS air-sea flux climatologies and the NCEP reanalysis data-set. In the limit of no diabatic mixing and at a steady state, these air-sea flux estimates of water-mass transformation are equivalent to estimating the residual circulation in the upper ocean. Second, an idealised model of an mixed layer and adiabatic thermocline for a channel is used to illustrate how changes in mechanical forcing can modify the air-sea heat flux and, thus, alter the residual circulation, as well as the relative contributions of Ekman and eddy transport, for different freshwater forcing scenarios and for different eddy parmeterizations. The model study highlights how buoyancy forcing need not always be the rate limiting process and instead mechanical forcing can become important in modifying the residual circulation. This Southern Ocean example is analogous to how air-sea heat fluxes in the shelf seas are altered by tidally-driven mixing and how the residual circulation in the stratosphere is controlled by the breaking of gravity waves.
The role of mesoscale eddies in the rectification of the Southern Ocean response to climate change
Riccardo Farneti, Tom L. Delworth, Anthony J. Rosati, W. Anderson, H.C. Lee, R. Pacanowski, R. Zang and F. Zeng (NOAA/GFDL)
The ability to represent the oceanic response to changes in mechanical and buoyancy forcingis of fundamental importance for understanding the climatic impact of increasing concentrations of atmospheric greenhouse gases. The Southern Ocean is a key player in the Earth’s climate for its importance in the global ocean circulation and water mass formation, interbasin connections and air-sea exchanges of heat, freshwater and tracer gases. Mesoscale oceanic eddies are believed to have a crucial role in the dynamical and thermodynamical adjustment of the Southern Ocean but, so far, eddy fluxes have only been parameterised in coarse-resolution climate models. We present simulations from a high-resolution global coupled model, the GFDL CM2.4, and compare the results with a coarse version of the same model (CM2.1) under climate change scenarios and idealised Southern Hemisphere wind changes. Compared to the case in which eddies are parameterised, the (marginally) eddy-resolving integrations show that eddies act as a buffer to atmospheric changes and the magnitude of the oceanic circulation response is greatly reduced. Changes in the eddy-induced circulation and associated poleward eddy fluxes partially compensate for the enhanced equatorward Ekman transport, leading to weak modifications in isopycnal slopes and ACC transport. Differences in atmospheric response when ocean eddy dynamics is active will also be discussed.
Ocean heat advection in the South-East Pacific in a high-resolution coupled GCM
Thomas Toniazzo (Uni. Reading), Roberto Mechoso (UCLA), Len Shaffrey (Uni. Reading), and Julia Slingo (Uni. Reading/Met Office)
We present an analysis of the oceanic heat advection and its variability in the Southeastern tropical Pacific as simulated by the global coupled model HiGEM, which has one of the highest resolutions currenly used in long-term integrations. The simulated climatology represents a temperature advection field arising from transient small-scale (<450 km) features, with structures and transport consistent with estimates based on observational data. The transient structures are very persistent (>4 months), and significantly affect the local oceanic heat budget on scales of several hundreds of km and over periods of over a year. While several different mechanisms may be responsible for the temperature advection by transients, including the ENSO and anomalous ventilation from the extra-tropics, the most significant component is associated with vortices embedded on the large-scale, climatological salinity gradient associated with the intrusion of fresher water from the Southern Ocean penetrating north-westward beneath the tropical thermocline.
The Antarctic Circumpolar Current across the AR4 models towards a common denominator
Till Kuhlbrodt (Uni. Reading)
The coupled climate models in the IPCC AR4 database are compared with respect to the representation of the Antarctic Circumpolar Current ACC. The motivation for this study is to understand why there is such a wide range of modelled ACC strengths from one fourth to about twice the observed volume transport through Drake passage. Moreover from the twenty-five models no statement can be made about how the ACC will develop toward the end of this century. Some models show an increase others a decrease or no trend at all. Given the very important role of the circulation in the Southern Ocean for the global oceanic uptake of heat and carbon dioxide it seems desirable to be able to make such a statement as could be done in the AR4 for the Atlantic meridional overturning circulation. Therefore we investigated which physical processes play a consistent role across the models. Candidates for these processes are the wind stress field water mass formation and model resolution. Eddy-driven transports are represented by the Gent & McWilliams 1990 parameterization in the vast majority of the AR4 models and their role is addressed in this study too.
Weak Along-Isopycnal and Strong Vertical Mixing in the Southern Ocean Interior
Jan Zika, Trevor J. McDougall, and Bernadette M. Sloyan (CSIRO)
The strength and structure of the Southern Hemisphere Meridional Overturning Circulation SMOC is related to the along-isopycnal and vertical mixing coefficients K and D respectively by analysing tracer and density fields from hydrographic data. The total transports across temperature contours on isopycnals inferred from both tracer and density distributions suggest a ratio of K to D of order 2x106 on the central layers of Upper Circumpolar Deep Water UCDW. Given existing estimates of the Overturning the circumpolar average K is found to be 300/-150m2s-1 suggesting K is much weaker in the interior than near the surface and D is estimated to be 10-4 /-0.5x10-4 m2s-1 consistent with previous estimates for the Southern Ocean. To explore spacial variation of K D and the overturning we develop and validate a new inverse method the Tracer Tube method. In the Tracer Tube method the flow across lines of constant tracer and density Tracer Tubes is related to a geostrophic streamfunction. Points sharing the same temperature and salinity such as along the Antarctic Circumpolar Current are connected via Tracer Tubes and these points are connected in the vertical via thermal wind. Using the Tracer Tube method along-isopycnal and vertical mixing processes as well as the mean and eddy induced circulations may be accurately inferred from the mean hydrography.
Eddy-induced mixing in the Southern Ocean
Alberto Naveira Garabato (NOC)
The character and structure of eddy-induced isopycnal mixing in the Southern Ocean are determined through the analysis of observations of thermohaline finestructure and altimetric variability within a mixing length theoretical framework. It is shown that typically the rate of eddy-induced mixing is strongly reduced in the upper layers of the frontal jets of the Antarctic Circumpolar Current ACC jets relative to their surroundings. A quasi-geostrophic theory of eddy-induced mixing in the presence of a mean flow is presented and used to interpret the observed patterns in terms of mixing suppression by the mean flow. It is further shown that the characteristic barrier behaviour of upper-ocean ACC jets breaks down at a few special sites. There significant departures from parallel flow appear to mark the onset of a distinct mixing regime in which jets become leaky. The dynamics underlying the existence of these leaky jet segments and their implications for Southern Ocean circulation are briefly discussed.
The Role of Internal Waves and Turbulence in the Meridional Overturning Circulation of the Southern Ocean
Maria Broadbridge (NOC), Alberto C. Naveira Garabato (NOC), David A. Smeed (NOC), David P. Stevens (UEA) and Raffaele Ferrari (MIT)
A series of numerical experiments of an idealized flow in a zonal channel are diagnosed to study the role of interior diabatic processes, such as the breaking of internal waves generated by bottom topography, in the Meridional Overturning Circulation (MOC) of the Southern Ocean. The experiments are conducted using the MIT General Circulation Model (MITgcm), configured to simulate an Antarctic Circumpolar Current (ACC), whilst incorporating various plausible representations of both adiabatic and diabatic forcing terms. The present understanding of the Southern Ocean MOC puts an emphasis on the role of direct wind forcing and mesoscale eddies in driving a purely adiabatic circulation across the ACC, which significantly influences the strength and structure of the global MOC. Here we investigate the extent to which interior diabatic forcing may provide an additional leading order term in the Southern Ocean MOC that is unaccounted for by current theories and numerical models.
Bottom Water Formation in the southern Weddell Sea and Western Ross Sea
Qiang Wang (AWI)
The Antarctic Bottom Water AABW is an important ingredient of the global thermohaline circulation. Two of the major precursors to AABW the southern Weddell Sea and the western Ross Sea density-driven overflows are studied using the Finite Element Ocean circulation Model FEOM with varying resolution. The characteristics of Filchner overflow originated from Ice Shelf Water and the influence of submarine ridges are investigated. The center of the descending dense water is found at depth greater than 2500 m after only 200 km along-slope distance. Most significant mixing and entrainment also takes place within this short distance. The conversion rate from ISW to newly formed Weddell Sea Bottom Water WSBW referred to potential temperature less than -0.8 °C is found to be 2.2. The transport of WSBW does not show significant changes after 400 km pathways but conversions between different water classes are still active. Trains of cyclonic eddies are observed along the plume path. Two ridges on the slope of the southern Weddell Sea can influence the overflow by both steering and increasing mixing. The dynamics of the western Ross Sea overflow derived from High Salinity Shelf Water is illustrated with the plume thickness and path mixing processes and eddies. Our preliminary results show that active eddies are not formed near to the shelf break but on the mid-depth slope further downstream contrary to the finding in the southern Weddell Sea. The reason can be the stabilization effect of the steep slope near to the shelf break.
Meso-scale eddy processes in North Pacific Subtropical Mode Water formation
Ivana Cerovecki (SCRIPPS)
Using the results of an eddy resolving ocean numerical model simulation we analyze the role of ocean mesoscale processes in North Pacific Subtropical Mode Water STMW formation in the years 1996-2003. STMW formation is estimated using a Walin 1982 type analysis and compared to STMW volume changes in the ocean interior. The results suggest that STMW formation cannot be explained solely in terms of the overlying atmospheric conditions but also strongly depends on advection of Ertel potential vorticity PV. When the Kuroshio is in an unstable state eddy fluxes of PV advect high PV from the northern flank of the Kuroshio in the region of NPSTMW pool. The eddy buoyancy flux divergence in the surface diabatic layer tends to oppose the air-sea buoyancy fluxes thus reducing STMW formation rates. Lateral induction through the sloping mixed layer base plays an important role in STMW destruction. Overall eddy processes play an important role in STMW formation formation rate estimates using snapshots or daily averaged data are substantially different from the formation rate estimates obtained using the same data but averaged over a month which averages out eddy processes and precludes following instantaneous isopycnals.
Toward a North Pacific simulation resolving submesoscale structures
Hideharu Sasaki (JAMSTEC)
Recent observations such as satellite observed SST and ocean color capture the submesoscale structures. Modeling studies with idealized setting e.g. Levy et al. 2001 Patrice et al. 2009 also simulate the submesoscales. Intense ageostrophic circulation and large vertical velocities are simulated in the models that resolve submesoscales and at this scale Rossby and Richardson numbers become O1 in localized regions. Such submesoscale structures could influence large oceanic fields in the surface and sub-surface. Biological fields could be also affected by small-scale nutrient injection triggered by submesoscales e.g. Levy et al. 2001. In the next generation of OGCMs that simulate realistic basin-scale circulations upper-layer submesoscales with intense vertical motion need to be resolved or should be parameterized. Motivated by recent development we made an attempt to conduct a high-resolution North Pacific simulation with 1/30 degree horizontal resolution using OFES OGCM for the Earth Simulator Masumoto et al. 2004 Komori et al. 2005 based on MOM3. A preliminary spin-up integration for 4 months has shown the ability to produce fine-scale structures including meso- and submeso-scale structures. Intense vertical motions characterised by submesoscales are also found from surface to mid-depth which is similar to those in the idealized simulation by Patrice et al. 2008. This preliminary result shows that the OFES with 1/30 degree horizontal resolution could simulate submesoscale structures in a realistic configurations that include basin scale circulations and mesoscales. This successful trial encourages us to extend the climatological spin-up simulation. We also plan to simulate biological fields using the OFES with ocean biological model embedded.
Diagnosis of eddy fluxes in a hierarchy of high resolution models of the North Pacific
Hiroyuki Tsujino (MRI)
Eddy fluxes of potential vorticity PV salinity and thickness in a hierarchy of eddy-active z-coordinate models of the North Pacific are evaluated at three isopycnal layers around the thermocline. The eddy-active models differ in their horizontal resolution about 18 km 6 km and 2 km. The highest resolution 2 km model has the best skill in reproducing oceanic structures while significant improvements are found in the transition from the 18 km to the 6 km model. Thus the main analyses are focused on the 6 km model. Motivated by the tendency of diffusivity based on a mixing length approach to be nearly isotropic in mid-latitudes the eddy fluxes are decomposed into the down gradient direction and the direction perpendicular to it. Divergent components of eddy fluxes are largely down the gradient of the time mean fields except for the small gradient region and the seasonal thermocline. The diffusion coefficients for the down gradient fluxes are similar for conservative tracers PV and salinity and seem to be scaled with a factor of about three by those based on a mixing length approach. The magnitude of down gradient diffusion coefficients increases with the model resolution. The difference between 18 km and 6 km are far larger than that between 6 km and 2 km indicating that the roles of mesoscale eddies could be well represented by the 6 km resolution. Eddy thickness fluxes bolus velocity are correlated well with eddy potential vorticity fluxes indicating that they could be parameterized as the down gradient fluxes of potential vorticity. The magnitude of the skew-diffusion part is comparable to the diagonal down-gradient part and its spatial distribution reflects the eddy induced component of the mean velocity fields.
Modification of gyre circulation by sub-mesoscale physics
M. Lévy (LOCEAN-IPSL), P. Klein (LPO-IFREMER), A.-M. Tréguier (LPO-IFREMER), D. Iovino (LOCEAN-IPSL), G. Madec (LOCEAN-IPSL), S. Masson (LOCEAN-IPSL) and K. Takahashi (ESP)
We analyze simulations of an idealized double-gyre circulation at coarse (1°), mesoscale (1/9°) and submesoscale (1/54°) resolutions, with seasonal wind and thermohaline forcings corresponding to a fixed atmospheric state. Focus is on the impacts of the sub-mesoscale dynamics (through the strengthening of the mesoscale eddies) on the equilibrated dynamical and density fields. Thus the experiments were run over the multi-year time required to equilibrate the mean circulation and the mean structure of the ventilated thermocline.
One original impact concerns the emergence of a regime of alternating zonal jets in the western part of the basin, particularly intense between the latitude of the free jet and the latitude of zero wind stress curl. Another impact is a decrease of the winter mixed-layer depth and the emergence of a restratification of the upper layers. These impacts have important consequences on, both, the gyre-scale density structure and the meridional heat transport. Regarding the gyre-scale density structure, we find that the wind-driven subtropical gyre is deeper at high resolution, due to the rectifying effect of eddy fluxes that is strongly underestimated in coarser resolution simulations. When the submesoscales are resolved, the effect of turbulence on the thermocline is to steepen isopycnal slopes instead of simply flattening them as assumed in eddy parameterizations of baroclinic instability. Stratification does not vary monotonically as the model grid is refined: a stratification decrease within the internal thermocline is observed from coarse to mesoscale resolutions, but taking into account the submesoscales leads to a significant stratification increase. The variation of the meridional heat transport is opposite to that found in many realistic simulations: the heat transport decreases at high resolution because the circulation becomes more zonal.
Impact of submesoscales appear to reduce the Meridional Overturning Circulation in the North - because of the restratification of the upper layers - and to locally intensify it at the latitudes of the western boundary current - because of the emergence of the zonal jets.
Tropical instability waves and their impact on the tropical atmosphere in a high resolution coupled climate model
Len Shaffrey (Uni. Reading)
HiGEM is based on the latest climate configuration of the Met Office Unified Model HadGEM1 but with the horizontal resolution increased to 1.25 x 0.83 degrees in longitude and latitude for the atmosphere and to 1/3 x 1/3 degrees globally for the ocean. Centennial length integrations of HiGEM are being used to explore questions about how higher resolution can improve the representation of regional climate and weather. Since HiGEM has a high resolution atmosphere coupled to a high resolution ocean it can simulate local coupled air-sea interactions on relatively fine scales. One of the scales that is resolved by HiGEM is the interaction of Tropical Instability Waves TIWs with the tropical atmosphere. Results will be presented showing that the structure of the atmospheric response to the TIWs is very similar to that seen in QuikSCAT imagery with regions of windstress divergence seen along the cross-wind SST fronts. This suggests that HiGEM is capable of simulating the coupled small-scale interactions between oceanic TIWs and the tropical atmosphere. One important question raised by these results is how the interaction between TIWs and the atmosphere in HiGEM might be parametrised in a lower resolution coupled climate models.
4DVAR data assimilation and Adjoint-based sensitivity analysis in an eddy resolving ocean model of the Peruvian Current System
Aneesh Subramanian (SCRIPPS), A. J. Miller (SCRIPPS), B. D. Cornuelle (SCRIPPS), and E. Di Lorenzo (Gerogia Inst. Tech)
Peruvian Current system plays a key role in the climate variability of the Eastern Tropical Pacific and also has teleconnections to other regions of high climate impact like the Tropical Pacific. This region is highly influenced by the strong wind forcing and the oceanic circulation is characterized by strong eddy activity with short zonal scales. The highly active primary production in this region with a very strong concomitant upwelling is of great importance to the surrounding economies. Hence, we use a high resolution regional ocean modeling system (ROMS) to study the sensitivity of the circulation in this region to wind forcing and meridional advection. Sensitivity of the eddy kinetic energy to wind forcing is also studied to exhibit their influence on its seasonal and interannual variability. Sea surface temperature, height and subsurface temperature and salinity data measured from CTD casts are assimilated via 4DVAR to generate an estimate of the current circulation for a period of two months in 2008. We present the methods and initial results for the assimilation.
CABARET in the ocean gyres: a novel high-resolution computational method for fluid mechanics
Sergey A. Karabasov (Uni. Cambridge), Pavel S. Berloff (Imperial College), and V.M. Goloviznin
In many aspects mesoscale oceanic eddies, operating on the lengthscales of O(1-100) km are analogous to the cyclones and anticyclones that constitute the atmospheric weather phenomenon. The problem of resolving these eddies in a dynamically consistent way is very important for ocean modelling and, therefore, for global climate predictions. For achieving high Reynolds number (Re) simulations, which are one of the goals of the ocean modelling, the models have to resolve all important scales of motion. At present this is, of course, too computationally expensive in practice and parameterizations of poorly resolved/unresolved scales with simple subgrid-scale (SGS) models must be used. Even without the complications due to SGS modelling, the problem of dealing with spurious numerical errors in the solution is very important because of the inverse energy cascade, typical of quasi-2D turbulence, that facilitates influence of the smaller scales on the larger scales of motion. This influence results in the spurious effect of the numerical phase and amplitude errors on the large-scale circulation.
A new high-resolution Eulerian numerical method is proposed for modelling quasigeostrophic ocean dynamics in eddying regimes. The method is based on a novel, second-order non-dissipative and low-dispersive conservative advection CABARET scheme. The properties of the new method are compared with the properties of the conventional and the so-called “high-resolution” Eulerian methods for a range of benchmark linear advection and gas dynamics problems. Then the CABARET method is applied to the classical model of the double-gyre ocean circulation. In turbulent regimes, the new method significantly accelerates convergence of the numerical solution and, thus, permits credible numerical simulations to be obtained on much coarser computational grids.
